1. Field of the Invention
This invention relates to an image pickup apparatus having an electronic zooming function for electronically increasing the magnification of an image of an object.
2. Description of the Related Art
Recently, video apparatuses such as video cameras and integrated camera/video recorders have been remarkably improved and the development of this kind of video apparatus having various functions and improved operability has been promoted. For example, automatic focusing mechanisms are presently provided as standard equipment, and electronic zooming functions for electronically increasing the image magnification by processing an image signal are now being introduced to realize large-magnification zooming without using a large optical zooming lens.
The automatic focusing method mainly used in conventional automatic focusing systems is a method in which a signal formed of high-frequency components, edge components or the like and representing the sharpness is extracted from an image pickup signal to determine a focal state. This method is characterized in that an object can be focused no matter what the distance to the object, and that there is no need to provide a special optical system and a sensor for detecting the focus.
However, with respect to the construction of conventional image pickup apparatuses having an electronic zooming device and an automatic focusing system using an image pickup signal, a problem described below is encountered.
That is, an electronic zooming apparatus conventionally used is arranged to electronically increase the magnification of an image of an object by intermittently reading out an image pickup signal from an image pickup device at a transfer rate determined by the magnification of the object image, by forming an interpolation signal from the read-out image pickup signal, and inserting the interpolation signal in the read-out image pickup signal. In an image pickup apparatus constructed by incorporating an automatic focusing system with this zooming apparatus, an image pickup signal processed to electronically increase the magnification by such interpolation/insertion is used as a signal for determining focused/non-focused states in an automatic focusing circuit.
Accordingly, a signal which is interpolated but thinned-out in a horizontal direction is used to determine focused/non-focused states, and a particular frequency component or edge component extracted from such a signal is different from the corresponding frequency component or edge component extracted from the original image pickup signal. There is therefore a problem of occurrence of a deterioration in the automatic focusing accuracy or a malfunction.